The present invention relates generally to improvements in the treatment of synthetic organic polymeric resins and it relates more particularly to an improved method for the production of foamed or cellular plastics.
Cellular plastics, particularly of the closed cell type, are widely used in many fields by reason of their high thermal insulation properties, low cost, easy shaping and machinery and wide range of physical properties. They are generally water and chemical resistant and with proper additives weather resistant and may be rigid or of any desired degree of flexibility.
A process for producing cellular products from synthetic organic polymeric resins, particularly polyvinyl chloride, is described in U.S. Pat. No. 2,576,749 granted Nov. 27, 1951 to P. J. Carpentier et al. In accordance with the aforesaid process a dry homogeneous mixture of a polymeric resin, such as polyvinyl chloride, a heat activated blowing agent, and a water activated blowing agent reactant are deposited in a receptacle and placed and restricted in a high pressure hydraulic press in which the mixture is heated to the gelation and fusion temperature of the resin and the activation temperature of the thermal flowing agent or gas generator. The heating step is conventionally performed with the mixture containing forms alternating with heat media circulating plates stacked between the platens of a large hydraulic press which can apply high pressures since the pressure of the gas generated by the blowing agent of the order exceeding 1800 pounds per square inch. Following the fusion of the resin and the uniform generation of small gas containing cells the forms are cooled and the cooled small pore cellular slabs or embryos are removed and subjected to a post treatment which includes exposing the embryo to water steam or moisture to react with the water activated blowing agent which, if the embryo is heated to its plastic state, expands the embryo by an amount depending on the amount of blowing agents in the mixture, the amount of water and the heating of the embryo and some degree of hardening of the resin body, if desired.
While the above process produces a highly satisfactory product with a wide range of properties, such as rigidity, flexibility, density and other characteristics, it possesses numerous drawbacks and disadvantages. It is an expensive process, requiring bulky, awkward and expensive equipment, is of little versatility and adaptability, highly time and skilled labor consuming, and otherwise leaves much to be desired.